The hydraulic circuit of a work machine, such as an excavator or a loader, typically includes a pump, a valve, and a hydraulic actuator in fluid communication. The hydraulic actuator may be a hydraulic cylinder, a hydraulic motor, or another hydraulic device supplying motive power to a work implement or drive train of the machine. When an operator of the machine actuates a valve by, for example, manually moving a lever, pressurized hydraulic fluid flows from the pump to the hydraulic actuator through the valve to move a work element of the hydraulic actuator, such as a piston in a hydraulic cylinder. This movement of the work element results in movement of an implement or a linkage assembly, such as a bucket, a boom, or a stick, coupled to the work element. For example, when a boom is to be raised, the operator moves the lever associated with the hydraulic circuit for the boom to open or close a valve in the circuit and to thereby pressurize an appropriate chamber of the hydraulic actuator in the circuit. Generally, in a work machine, there are multiple hydraulic circuits to control each of the work implements and linkage assemblies independently. Typically, each hydraulic circuit includes a valve and a hydraulic actuator.
In normal operation, the implement or linkage assembly is often brought to an abrupt stop after performing a given function. For example, a boom of a loader being lowered toward the ground may come to an abrupt stop when it hits the ground. Similarly, the boom may be raised to the upper end of its range of motion and come to an abrupt stop. When an implement or linkage assembly comes to an abrupt stop, significant forces are absorbed by the machine, resulting in an increased machine failure and maintenance. Also, such an abrupt stop causes a jerky movement of the machine and discomfort to the machine operator. A similar condition may occur when a moving speed of an implement or linkage assembly is significantly accelerated or decelerated.
To solve these problems, some hydraulic systems control stopping of the hydraulic cylinder at the end of stroke by hydraulic snubbers enclosed within the cylinder. Also, some systems provide a “snubbing” or “feather catch” operation to electronically modulate the lowering speed of an implement during gravity assisted operations. U.S. Pat. No. 5,727,387, for example, discloses an implement control apparatus for a work machine. The apparatus includes multiple hydraulic cylinders in fluid communication with a pump and valves. The apparatus senses a joystick position and generates an electrical valve signal proportional to ajoystick position signal. While the apparatus disclosed in U.S. Pat. No. 5,727,387 controls hydraulic fluid flow through a valve based on a joystick position signal, it does not control hydraulic actuators for a work machine by sensing a linkage position and a command signal and selecting a moving rate of the actuators.
Also, U.S. Pat. No. 6,257,118 discloses an apparatus for controlling a hydraulic cylinder. The apparatus includes an electronic controller that receives an operator command signal and a position signal, determines a velocity of an element, and determines a limit value in response to the velocity of the element. The apparatus, however, does not control hydraulic flow to a hydraulic actuator based on a moving rate of a valve.
Thus, it is desirable to provide a hydraulic flow control system that provides flexible control of actuator acceleration, deceleration, and response based on operational conditions. The present invention is directed to solving one or more of the problems associated with prior art designs.